For dental impression compositions, two-component compositions are used which are contained in tubular bags. These bags are held in a cylinder inside a dispensing unit. The composition is pressed out by the action of a plunger and dispensed through a mixer (EP-A 492 413). In a known unit of this type (EP-A 541 972), each tubular bag is provided at its front end with a mouthpiece which cooperates sealingly with a complementary opening on the end face of the cylinder. The end face of the cylinder forms a head piece in which there are channels for guiding the components to attachment pieces to which the mixer can be connected. The cylinders and the head piece are fixed elements of the dispensing unit. If one wishes to exchange the bags containing the components, then the bags are removed from the cylinders holding them, and other bags are inserted, during which procedure care has to be taken to ensure that their mouthpieces have a sealed connection to the openings provided in the head piece. It can easily happen that the sealed connection is not obtained, or is not obtained directly upon insertion, and that some of the components pass into the unit. It can also happen that air is trapped in the composition and leads to errors during use, or that components are erroneously mixed up.